Leaflets of Western Botany 5: 113. 1948.
Herbs, annual, cespitose, 0.2–1.2 dm. Culms to 100, (0.2–)0.4–0.8 mm diam.. Leaves 1/3–3/4 height of plant. Inflorescences terminal clusters, each with 1–7 flowers; bracts subtending inflorescence 2–8, round to acutely ovate, inconspicuous, 0.7–2.4 mm, membranous. Flowers: tepals reddish to black, narrowly lanceolate to oblong, 2–4.6 × 0.5–1.4 mm; inner series usually slightly longer than outer, apex obtuse to acuminate; stamens 3, filaments 0.5–1.4 mm, anthers 1–3.2 mm; style persistent, 1.4–4 mm, stigma 1.3–4 mm. Capsules reddish, 3-locular, globose to ellipsoid or oblong, 2–4.5 × 1–3 mm, nearly equal to sepals. Seeds ovoid, 0.35–0.45 mm. n = 16.
Phenology: Flowering and fruiting in spring.
Habitat: Margins of vernal pools
Elevation: 0–500 m
Plants from Butte and Calaveras Ccounties, California, tend to be smaller and consistently develop only one flower per culm; they may be recognized as Juncus leiospermus var. ahartii Ertter.
Of conservation concern.